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Latest Developments in GPC Analysis of

Adhesive and Sealant PolymersMark Pothecary PhD

Americas Product Manager

Malvern Instruments

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Molecular weight

▪ The most fundamental molecular property that controls a polymer’s behaviour is its

molecular weight

▪ A polymer sample normally contains a range (distribution) chains of varying molecular

weight

PS

PMMAPVC

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Gel-permeation chromatography

▪ GPC (also known as size-exclusion chromatography, SEC) has long been used as a key

tool for measuring molecular weight

▪ GPC separates macromolecules in solution

▪ It is based on the principle of separating the molecules according to their size in a

chromatographic column

▪ After the column, the separated molecules can be analysed by one or more detectors

Sample

Solution

Component B

Component A

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Conventional calibration-RI or UV detector

measuring concentration

▪ Conventional calibration takes some standards of known molecular weight to create a

calibration curve

▪ The unknown sample is then measured and its elution volume compared with the

standards

▪ This has many limitations including, most-notably, that if the standard and sample

polymers are different, the molecular weight will be only a relative estimate

PMMA95k

Peak RV - (ml) 17.98

Mn - (kDa) 40.10

Mw - (kDa) 88.02

Mz - (kDa) 151.91

Mw / Mn 1.129

IV - (dl/g) N/C

Rh(w) - (nm) N/C

Rg(w) - (nm) N/C

Polystyrene 400k as part of calibration curve PMMA is measured against this curve

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Multi-detector GPC

▪ GPC with static light scattering and intrinsic viscosity can measure absolute molecular

weight and intrinsic viscosity, independent of elution time, structure and chemistry

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Static Light Scattering – Molecular

Weight

▪ Measures the overall intensity of light

scattered by a sample at known

concentration, from which the

molecular weight can be calculated at

each data slice using the Zimm

equation

CAPMR

KC

W22

1

Where K = constant; c = concentration; Rθ = Rayleigh ratio; Pθ = angular dependent term; A2 = 2nd virial coefficient

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How does intrinsic viscosity relate

to structure?

▪ IV has the basic units

– dL/g

▪ IV is inversely proportional to

molecular density

– IV ∝ 1/density

▪ We can look at structure in these

terms:

– IV ∝ Volume/mass

▪ Which of these two molecules with

the same mass occupies the

largest volume of space?

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Mark-Houwink plots

▪ The Mark-Houwink plot is used to compare the different structures of polymers

▪ It is often used to study conformation and branching

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The multi-detection pyramid

δ Refractive

Index

UV

absorbance

Light scattering

intensityδ viscosity

Concentration

Molecular weight

(SLS)

Intrinsic

viscosity

Hydrodynamic

radius (Rh)

Mark-Houwink

parameters

Branching

Composition

Radius of

gyration (Rg)

Directly

measure:

Directly

calculate:

Indirectly

calculate:

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Adhesives and sealant applications

▪ Methacrylates, epoxies, polyurethanes, cellulose derivatives (methyl

cellulose), silicones and many others

▪ Polymers, often with low molecular weights

▪ Few molecular weight standards

▪ Molecular weight, polydispersity and structure likely to affect:

– Adhesion strength

– Peel and shear strength

– Flow

– Toughness (extension and compression characteristics)

→Low molecular weights tend to be brittle

→Entanglement and toughness increase with MW

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Epoxies & resins

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Bisphenol A (MW = 228)

▪ Monomer for epoxy

▪ Pure sample has large RI signal with very low LS signal

› Mobile Phase: THF Columns: 2 X T2500 Flow Rate: 1 mL/min

› Temperature: 30C Injections: 100 µL of 3 mg/mL dn/dc: 0.195

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Bisphenol A Diglycidyl Ether (MW = 340)

› Mobile Phase: THF Columns: 2 X T2500 Flow Rate: 1 mL/min

› Temperature: 30C Injections: 100 µL of 5 mg/mL dn/dc: 0.140

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Epoxy oligomers

▪ With the right columns, oligomers can be resolved and their individual

molecular weights measured

RILSIV

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Methacrylates

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PMMA processing

▪ Sample of PMMA run through 5 capillary rheology measurements

simulating cycles of moulding and re-moulding– (each line is overlay of 4 injections!)

RILALSIV Cycle 1 - 5

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PMMA processing

▪ The changes in molecular weight distribution are significant

▪ The amount of high molecular weight material actually increases

– (each line is overlay of 4!)

Cycle 1 - 5

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PMMA processing

▪ Molecular weight increases

with each processing cycle

▪ Increasing polydispersity will

affect toughness

▪ …could mean greater fragility

or improved robustness…

▪ …means greater product

variability and lower quality

and value

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PMMA processing

▪ After processing, there are clear differences in the structure of PMMA as well

as the molecular weight– (each line is overlay of 4 injections!)

▪ Suggests cross-linking

Cycle 1 - 5

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PMMA processing– Rheology data

▪ Capillary rheometer results show some small differences in viscosity at

low shear but similar viscosities at higher shear

▪ A bulk material measurement is not enough to see the underlying

differences…in this case

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Polymer processing

▪ Different polymers respond differently to processing

▪ Understanding how polymer behaves during processing is key to a robust

process

PS PMMA

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Polycaprolactone

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Polycaprolactone (PCL)

▪ Biodegradable polyester

▪ Low melting point around 60°C

– Easily moulded simply by immersing in hot water

▪ Used in the manufacture of polyurethanes

▪ Plasticizer additive for PVC

▪ Controlled release/drug delivery polymer with slower degradation times than

PLA

– Biodegradable implants

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Polcaprolactone degradation

▪ SEC measurements of PCL

– Raw/virgin – has the highest molecular weight

– Extruded at 80°C – shows some degradation

– Extruded at 60°C in the presence of CO2, which acts as a molecular lubricant allowing similar processing

at lower temperatures – protects sample from some of the degradation

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Polcaprolactone degradation

▪ At first glance, the Mark-

Houwink plots all look to

overlay well

▪ At closer inspection there

is a tiny but very

repeatable difference in

the plots

▪ Possible explanations

– Changes in branching levels due

to extruding

– Changes in moisture content in

absence of CO2

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Polycaprolactone rheology

▪ Small viscosity changes in response to extruding– Virgin PCL has highest viscosity

– Decrease in viscosity following extrusion and concurrent decrease in molecular weigh

– Extrusion in presence of CO2 can be performed at lower temperature and protects sample from

some degradation

PCL

PCL extruded + CO2

PCL extruded

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Silicones

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Silicones

▪ Three silicone samples

Sample

Injection 1 2 1 2 1 2 1 2

Mw (Da) 10,335 10,164 10,983 11,407 11,656 11,761 11,096 10,875

Mn (Da) 4,375 4,141 5,981 6,857 6,696 6,924 5,630 2,695

IV (dl/g) 0.0782 0.0766 0.0733 0.076 0.0754 0.075 0.0755 0.0765

Rh (nm) 2.206 2.174 2.214 2.273 2.285 2.286 2.243 2.212

1 2 3 4

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Cellulose derivatives

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Cellulose derivatives

▪ Derivatives of cellulose are commonly used in all sorts of pharmaceuticals

– Eye drops

– Moisturizing creams

▪ Raw cellulose can be derivatised in different ways:

– Hydroxyethyl cellulose

– Hydroxypropyl methyl cellulose

– Hydroxypropyl cellulose

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Cellulose derivatives

▪ Hydroxybutylmethyl cellulose (HBMC)

▪ Hydroxypropylmethyl cellulose (HPMC)

▪ Carboxymethyl cellulose (CMC)

▪ Hydroxpyopyl cellulose (HPC)

▪ Methyl cellulose (MC)

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Cellulose derivatives

▪ Different cellulose

derivatives can be

compared by number and

on the Mark-Houwink plot

▪ These derivatives will have

different effect on (e.g.)

formulation viscosity

▪ This will depend on

molecular weight,

structure/branching and

level of derivatisation

SampleMn

(g/mol)

Mw

(g/mol)

Mz

(g/mol)[η] (dL/g) Rh (nm)

HEC 62,600 223,000 712,000 3.572 21

HPC 45,600 69,000 111,000 1.113 10.21

HPMC 98,300 306,000 692,000 7.085 29.85

HBMC 102,000 362,000 831,000 8.556 33.54

HBMC

HPMCHPC

HEC

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Summary

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Summary

▪ GPC is a key tool for separation and characterization of polymers

▪ Advanced/multi-detector GPC can be used to measure absolute

molecular weight, as well as structural aspects such as branching and

conformation, which will have significant effects on adhesive and

sealant properties and performance

▪ The latest developments in GPC detectors allow measurements of

lower molecular weight and lower dn/dc samples where light scattering

sensitivity is a key concern

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Thanks for listening

Mark Pothecary PhD

Americas Product Manager

Malvern Instruments